Lyme disease and human granulocytic ehrlichiosis (HGE), caused by Borrelia burgdorferi and Anaplasma phagocytophila respectively, are two common vector-borne illnesses in the United States. Both pathogens are transmitted to man by Ixodes scapularis ticks, and dual infections have been documented in the arthropod and vertebrate hosts. The purpose of this application is to determine whether co-infection of mice with B. burgdorferi and A. phagocytophila alters B. burgdorferi infectivity and the severity of murine Lyme arthritis. Our recently published report showed that the simultaneous experimental infection of mice with B. burgdorferi and A. phagocytophila increased spirochete numbers and the severity of joint inflammation. We will now explore this further by determining how the challenge dose of each organism and the timing of exposure of each pathogen influences the course of murine Lyme borreliosis. We will also determine whether these differences are observed when B. burgdorferi and A. phagocytophila are transmitted via tick bite rather than needle inoculation. Our published studies have also demonstrated that antibodies against B. burgdorferi genes that are expressed in vivo contribute to immunity against the spirochete and that B. burgdorferi gene expression can be influenced by the host immune response. We will now use spirochete DNA microarrays that we have developed to explore the hypothesis that B. burgdorferi gene expression in vivo is modified during co-infection with A. phagocytophila. We will identify B. burgdorferi genes that have altered levels of expression during co-infection and examine the role of these gene products in immunity against infection and the severity of joint inflammation. These studies will more clearly define the influence of B. burgdorferi and A. phagocytophila co-infection on the course of murine Lyme arthritis and explore the mechanisms by which dual infection alters spirochete infection. These efforts should increase our understanding of the importance of co-infection with these tick-borne agents on the outcome of disease and serve as a general model for how dual infection can influence host responses and pathogen gene expression. [unreadable] [unreadable]